1. Field of the Invention
The present invention relates to an acoustic transducer for use in a loudspeaker, a headphone and the like, and more particularly a composite type acoustic transducer of a piezoelectric type transducer and a flat drive type electrodynamic transducer. In the piezoelectric type transducer a diaphragm comprising a membrane made of high molecular piezoelectric materail and a pair of electrode layers applied on respective surfaces of the membrane is supported in a concave or dome-shape and is caused to vibrate due to shrinkage and stretch of the membrane in accordance with an audio signal voltage applied across the electrodes. In the flat drive type electrodynamic transducer a diaphragm or membrane which has a coil-like conductor applied thereon and is arranged in parallel to a magnetic field is caused to vibrate in a homogeneous or uniform phase over the entire surface of the membrane in accordance with an audio signal current supplied to the coil-like conductor.
2. Description of the Prior Art
Heretofore there have been proposed and designed various kinds of the flat drive type electrodynamic acoustic transducers and piezoelectric type acoustic transducers. For instance Japanese Patent Publication No. 10,420/70 and U.S. Pat. No. 3,674,946 disclose the flat drive type electrodynamic acoustic transducer including a number of rod-shaped permanent magnets. Further Japanese Utility Model Laid-open Publication No. 37,625/73 and U.S. Pat. No. 3,792,204 describe the piezoelectric type acoustic transducer comprising a dome-shaped piezoelectric membrane.
FIG. 1 is a cross sectional view showing an embodiment of the known flat drive type electrodynamic acoustic transducer of a kind disclosed in the Japanese Patent Publication No. 10,420/70. A membrane 1 made of resilient material such as polyester is supported by a pair of supporting frames 2a and 2b along its edge. Openings of the frames are covered with casings 3a and 3b having a number of small apertures 4a and 4b, respectively formed therein. On inside surfaces of the casings 3a and 3b are secured a number of rod-shaped permanent magnets 5a and 5b, respectively, in parallel to each other. As shown in FIG. 1 these magnets are so arranged that the magnets opposite to each other with respect to the membrane 1 have the same polarity, but adjacent magnets have different polarities so as to form magnetic fields in parallel with the membrane 1 as shown by an arrow A. On the surface of the membrane 1 is provided a coil like conductor 6 by, for instance evaporation, in such a manner that an electric current can pass through adjacent leg portions of the coil like conductor 6 in opposite directions. When the audio signal current flows through the coil like conductor 6 there is produced a force to drive or displace the membrane 1 in a direction perpendicular to the direction A due to an electromagnetic interaction between the current and the magnetic field. In this manner the flat drive type electrodynamic acoustic transducer can reproduce an acoustic wave in accordance with the audio signal.
FIG. 2 is a cross section for illustrating an embodiment of the known piezoelectric type acoustic transducer described in the Japanese Utility Model Laid-open Publication No. 37,625/73. The transducer comprises a base plate 7 having a number of apertures 8 formed therein, a diaphragm 9 secured to the base plate 7 along its edge by means of a securing frame 10, and a resilient member 11 arranged between the base plate 7 and the diaphragm 9. The diaphragm 9 is supported along the curved surface of the resilient body 11. The diaphragm 9 consists of a membrane 12 made of high molecular piezoelectric material and a pair of electrode layers 13 and 14 applied on respective surfaces of the piezoelectric membrane 12. The electrode layers may be applied by evaporation. When an audio signal voltage is applied across the electrode layers 13 and 14, the piezoelectric membrane 12 is caused to shrink and stretch in a direction shown by an arrow B in accordance with the audio signal and as a result the diaphragm 9 vibrates in a direction perpendicular to the direction B so as to produce an acoustic wave.
In the known acoustic transducers, since the driving force is produced at every portion of the membrane or diaphragm in a substantially same direction and thus the membrane vibrates in a homogeneous phase, the ideal piston motion of the diaphragm is realized and thus a reproduction characteristic of the transducer is superior to that of ordinary cone-type electrodynamic transducer. Particularly a distortion of the flat drive type electrodynamic transducer is materially smaller than the cone type electrodynamic transducer. Further the flat drive type electrodynamic transducer has a very flat sound level/frequency response.
However in the known flat drive type electrodynamic transducer as shown in FIG. 1 it is very difficult to produce a sufficiently large magnetic flux density due to its construction and thus an efficiency is rather low. Also in the known piezoelectric type transducer illustrated in FIG. 2 any piezoelectric material having a sufficiently high piezoelectric modulus could not be found and therefore the efficiency is also low. Further in the known transducers since the diaphragm is suspended under a certain tension an amplitude of vibration is rather small and a sufficient reproduction of lower frequency sound could not be attained. For instance, in the transducer shown in FIG. 1, the membrane 1 is always stretched in the direction A and thus there is always produced a force which limits the displacement of the membrane in the direction perpendicular to the direction A. This results in the decrease in the efficiency and vibration amplitude and therefore the reproduction characteristic in the lower frequency range might be deteriorated. Under the above circumstances an application of such transducers has been usually limited only to a headphone and a tweeter.